Project Summary/Abstract Large clinical outcome trials with inhibitors of excess neurohormonal activity in heart failure (HF) with reduced ejection fraction (HFrEF) patients have shown significant decreases in mortality. However, the effects of these same neurohormonal inhibitors in HF with preserved ejection fraction (HFpEF) patients have consistently failed to reach positive primary outcomes. The dichotomy in the responses of the patients likely results from differences in HFpEF versus HFrEF pathophysiology, and the absence of experimental HFpEF models that capture essential characteristics of this syndrome. Our contention, and the approach used in the proposed work, is that concomitant use of a large animal HFpEF model and a mouse model of diastolic dysfunction with preserved EF will enable us to document common effects of a putative therapeutic on the HFpEF phenotype, thus facilitating translation of our findings to the estimated 2.5 million humans currently suffering from HFpEF in the United States alone. Within the last year, work by the two PIs of this proposal was the first to illustrate a crucial role for a family of epigenetic regulatory enzymes, histone deacetylases (HDACs), in the control of diastolic dysfunction and HFpEF pathogenesis. The McKinsey laboratory showed that a small molecule HDAC inhibitor prevented diastolic dysfunction in rat and mouse models of diastolic dysfunction with preserved EF triggered by hypertension or aging. Strikingly, the work was the first to link impairment of myofibril relaxation to the development of diastolic dysfunction in rodents, as well as in humans with HFpEF. Furthermore, it was shown that HDAC inhibition improves relaxation of the heart by promoting myofibrillar protein acetylation, thereby speeding myofibril relaxation rates. The Houser laboratory has recently demonstrated similar beneficial effects of an HDAC inhibitor in a feline model that recapitulates many elements of HFpEF in humans. The studies proposed in this application would define a strategy for HDAC inhibition that provides the greatest therapeutic efficacy, setting the stage for a proof-of-concept Phase 2a clinical trial with an HDAC inhibitor in patients with HFpEF. Furthermore, the work would define which HDAC isoforms promote diastolic dysfunction, and expand our understanding of the cellular and molecular mechanisms by which HDAC inhibitors improve relaxation of the heart. Three independent specific aims are designed to significantly extend this new field of translational cardiac research, and test the overall hypothesis that increased HDAC activity contributes to the pathogenesis of HFpEF by promoting diastolic dysfunction via deacetylation of proteins that regulate myofibril relaxation, cardiac fibrosis and/or sarcoplasmic reticulum calcium uptake.